This application is based on Patent Application No. 2001-241367 filed Aug. 8, 2001 in Japan, the content of which is incorporated hereinto by reference.
1. Field of the Invention
The present invention relates to an optical filter, and more specifically, to an optical filter that splits or combines wavelength multiplexed optical signals.
2. Description of the Related Art
An optical filter that splits or combines optical signals according to wavelengths is a key device in a wavelength multiplexing optical communication system. In particular, an interleaver that can extract or insert every other of a plurality of optical signals spaced at optical frequency intervals of xcex94f can be combined with an arrayed wavelength grating or a dielectric multilayer optical filter to easily and economically provide a high-density wavelength multiplexing optical communication system.
Conventional interleavers are implemented in a lattice type configuration having a large number of Mach-Zehnder interferometers connected together. Such an interleaver is described in, for example, Oguma et al., xe2x80x9cFlat-passband interleave filter with 200 GHz channel spacing based on planar lightwave circuit-type lattice structurexe2x80x9d, Electronics Letters, Vol. 36, No. 15, pp. 1299 to 1300.
FIG. 1 shows the configuration of a conventional interleaver. The interleaver is composed of a substrate 100 and the following components formed on the substrate 100 and connected together in the following order: input optical waveguides 101a and 101b, a first optical coupler 102, a Mach-Zehnder interferometer consisting of a pair of optical waveguide delay lines 103a and 103b having an optical path difference 2xcex94xcfx84, a second optical coupler 104, a pair of optical wavelength delay lines 105a and 105b having an optical path difference xcex94xcfx84, a third optical coupler 106, and output optical waveguides 107a and 107b. 
The splitting ratio of the three optical couplers constituting the interleaver is set so that the first optical coupler is 10%, the second optical coupler is 70%, and the third optical coupler is 50%. Further, the optical path difference xcex94xcfx84 is set at 10 psec so that the free spectral range (period on an optical frequency axis) of transmission characteristics is 100 GHz.
FIG. 2 shows the transmission characteristics of the interleaver. These transmission characteristics are observed at a first and second output waveguides when an optical signal is input to a first input waveguide. Both transparencies form the first input waveguide to the first and second output waveguides have stop bands where the optical signal is prohibited and pass bands where the optical signals is allowed to go through in an alternative manner. Further, when light at a frequency is allowed to go to the first waveguide (the pass band), the light is not allowed to go to the second waveguide (the stop band), and vice versa.
As shown in FIG. 3, an optical multi/demultiplexer of 50 GHz channel spacing can be implemented by combining the interleaver 110 with optical multi/demultiplexers 111 and 112 of 100 GHz channel spacing. In general, it is more difficult to produce an optical multi/demultiplexer with a narrower channel spacing. However, an optical multi/demultiplexer with a substantially narrow channel spacing can be provided by using an optical multi/demultiplexer of a relatively wide channel spacing which can be easily produced and the interleaver 110. To avoid degrading the transmission characteristics of an optical multi/demultiplexer of a narrow channel spacing, the transmission characteristics of the interleaver must be such that for a pass band, transmittance is close to 1 over a relatively wide range of optical frequencies, and for a stop band, transmittance is close to 0 over a relatively wide range of optical frequencies.
However, the conventional interleaver has the following problems: first, the conventional interleaver has only two outputs but has more than two parameters to be set. Accordingly, adjustment of the parameters is very complicated. In the example shown in FIG. 1, coupler splitting ratio must be set for three locations, and phase must be set for two locations. Thus, a total of five parameters must be set.
Second, the conventional interleaver has delay lines arranged in series, so that if half wave plates are used to compensate for birefringence in the optical waveguides, a half wave plate must be inserted into each stage. As a result, excess losses increase. Further, to compensate for the dependence of the optical waveguides on temperature, material (hereinafter referred to as xe2x80x9ctemperature compensating materialxe2x80x9d) whose refractive index change with a temperature has an opposite sign to that of the optical waveguides must be inserted into each stage. As a result, excess losses increase.
Third, instead of realizing the flat characteristics of pass bands using a smaller number of stages, the conventional interleaver has non-linear phase characteristics. As a result, the pass bands have chromatic dispersion. Such chromatic dispersion may cause signal quality to be degraded when interleavers are used in a long-distance high-speed optical transmission system.
It is an object of the present invention to provide an optical filter which allows parameters to be easily adjusted, which allows birefringence and temperature dependence to be easily compensated for, and which is essentially free from dispersion.
To attain this objects, an optical filter comprises an input optical waveguide, a first optical coupler that splits an optical signal guided by the input optical waveguide into two parts, at least one optical splitting means connected to an output of the first optical coupler, two groups of optical waveguide delay lines (the minimum number of constituents of the group is 1) connected to outputs of the optical splitting means, or an output of the optical coupler and an output of the optical splitting means, at least one optical combining means for combining any lights from the two groups of optical waveguide delay lines, a second optical coupler having a symmetric power splitting ratio and connected to outputs of the optical combining means, or an output of the optical combining means and the optical waveguide delay line, and an output optical waveguide connected to an output of the second optical coupler, all of said components being formed on a substrate, and wherein delay time provided by one of the two groups of optical waveguide delay lines is set to be xcfx840+2nxcex94xcfx84+xcex1n (xcfx840 and xcex94xcfx84 are positive real numbers, n is an integer that varies with optical waveguide delay line, |xcex1n|xe2x89xa6xcex/u, xcex is a wavelength, and u is the speed of light propagating through the waveguides), and delay time provided by the other group of optical waveguide delay lines is set to be xcfx840+(2 m+1)xcex94xcfx84+xcex2m (m is an integer that varies with optical waveguide delay line, |xcex2m|xe2x89xa6xcex/u).
The above and other objects, effects, features and advantages of the present invention will become more apparent from the following description of embodiments thereof taken in conjunction with the accompanying drawings.